This invention relates to radiation detectors in general, and more particularly to detectors having a two-dimensional array of PIN diodes integrated on a common silicon substrate together with circuitry to rapidly read out data from diodes in which impinging radiation generates charge.
The development of silicon microstrip radiation detectors as disclosed, for example, by J. Kemmer (Nucl. Instr. and Meth., 169 (1980) 499 and 226 (1984) 89), J. England, et al. (ibid. 185 (1981) 43), U. Kotz, et al. (ibid. A235 (1985) 481) and E. Heijne (CERN Report 83-06 (Jul. 21, 1983)) has improved the accuracy of electronic particle detectors by more than an order of magnitude. Until recently, associated readout electronics required channel-to-channel spacings two orders of magnitude larger than that of the strips to which they were attached. The low noise VLSI readout chip described by J. T. Walker, et al. (Nucl. Instr. and Meth., 226 (1984) 200), G. Anzivino, et al. (ibid. A243 (1986) 153), C. Adolphsen, et al. (ibid. A253 (1987) 444) and C. Adolphsen, et al. (IEEE Trans. on Nucl. Sci., 33 (1986) 57) has a channel pitch of 47.5 microns and provides amplification, integration, calibration and output signal multiplexing.
While the development of the low noise VLSI readout chip solved important space and cost problems, there still remain other important efficiency and accuracy problems inherent in the prior art. The use of microstrips provides for the information to be collected on and travel along a strip that is typically 8 cm long. Accordingly, there is no way of detecting where the charge initially attached to the strip, as all information as to the charge's original position in the direction of the strip is lost. Thus, information available in the ionization path is immediately degraded when using a microstrip collection structure, with the result that the detector does not provide accurate position information.
Another problem associated with the use of strips is their large capacitances and low resultant voltages. FIG. 1 illustrates a structure including one channel of a silicon microstrip radiation detector with its attached amplifier. In FIG. 1, Cx is about 1 Pf/cm, or about 10 Pf (due mostly to adjacent typically 10 cm length strips), the amplifier capacitance is about 2 Pf, and the feedback capacitance is about 0.1 pF, and the total effective capacitance is relatively large, namely about 10 pF+2 pF+350.multidot.0.1 pF.apprxeq.47 pF.
Since voltage is exactly equal to the charge divided by the capacitance, that is, V=Q/C, and a typical charge collected by a detector is, Q=80 eh/.mu.m.times.300 .mu.m.times.1.6.times.10.sup.-19 coulombs, the resultant voltage is V=82 .mu.V, e.g., V=(24,000).multidot.(1.6.multidot.10.sup.-19)/(47.multidot.10.sup.-12). Therefore, because the voltage at the amplifier input is quite small, high gain, low noise amplifiers are required for a reasonably sized detector, where the strips are necessarily long. Furthermore, the dynamic input capacitance of the amplifier must be even larger to prevent the charge from remaining behind and inducing comparable voltage signals on adjacent strips.
Radiation damage is yet another problem associated with silicon detectors, which damage can result in increased diode leakage current. The initial diode current (as well as any increase therein) is generally proportional to the diode volume, and fluctuations in this current can cause noise. Such noise current fluctuations are greater for strip detectors because of their larger volume, than for pixel devices. Furthermore, a larger strip size results in larger capacitance, and for a given amount of signal charge Q, a smaller signal voltage .DELTA.V results. The smaller signal voltage causes any radiation damage induced noise increase to become more of a problem. Thus, accuracy and efficiency in silicon microstrip structures tend to be more readily and easily degraded by radiation damage.
In prior art semiconductor detectors, a number of problems are associated with the attached electronics. Most detectors fabricate the control and readout electronics on a separate integrated circuit ("IC") chip, which usually results in higher capacitance. The detector IC chip itself must be made with easily depleted, high resistivity silicon. Thus, difficulties arise if the electronics is simply placed on the detector chip. For example, drain to source punch-through in field effect transistors ("FETe") can occur, which can cause large source to drain currents to flow regardless of the input signal voltage. The signal charge to be detected may drift into the bottom of the electronics rather than into the input gate of an amplifying first stage transistor, causing signals to be missed and thus undetected. Alteration of the signal collecting fields in the bulk of the detector due to switching transients in the electronics can occur, causing errors in the measured location of the radiation that generated the signal charge.
In summation, there is a need for a radiation detector that can efficiently and accurately generate position information as to radiation generated charged particles. Preferably such detector should provide a collection structure that permits the position of the collected charge to be accurately measured. To promote higher detection signal voltage, the detector collection structure should provide small capacitance, thus reducing the need for high gain, low noise amplification, and reducing system noise in general. Further, there is a need for a radiation detector that is free from ambiguity resulting from near simultaneous radiation hits.
The radiation detector should be radiation hardened, and have enhanced resistance to transistor punchthrough, and should include detection and readout functions integrated on the same silicon substrate as the detector. Such detector should further provide an electrostatic shield between the readout electronics and the depleted substrate containing the signal charge collection field. There is a need for a radiation detector whose readout electronics circuitry can rapidly detect position information from pixels that have been hit by an impinging ionizing particle. Finally, such a detector should have a plurality of charge-generating and charge-collecting pixels, formed in a two-dimensional array of columns and rows.
The present invention provides such a radiation detector.